The present invention relates to a cylinder head gasket with different seal or reinforce coatings. In particular, in tightening the bolts when assembling the engine, the difference between the sealing pressure formed around the hole for the outer cylinder bore and that formed around the hole for the inner cylinder bore is made small to improve the sealing characteristic around the holes for the cylinder bores.
As shown in FIG. 7, a cylinder head gasket 11 with a plurality of holes for the cylinder bores for sealing between a cylinder head and a cylinder block includes holes 13 for the cylinder bores, oil holes 15, water holes 16, and bolt holes 17, 18. Also, in order to seal combustion gases generated in the combustion chambers, oil circulating through an oil gallery and water in a water jacket, the gasket 11 includes sealing means, such as beads 12 and seal rings 14, for providing adequate sealing pressures to the respective holes to be sealed.
Especially, since high temperature and high pressure combustion gases are generated around the holes 13 for the cylinder bores when the engine is operated, sealings around the holes 13 are especially important. Therefore, a metal plate superior in mechanical strength and durability is used for the gasket, and beads 12 for forming main sealing portions are arranged around the holes 13 for the cylinder bores to obtain high sealing characteristics by the beads 12.
Also, in the bolt holes 17, 18, head bolts (not shown) are inserted, and the gasket is sandwiched between the cylinder head and the cylinder block (not shown) and is tightened by the head bolts. The bolt holes 17, 18 are arranged to surround the holes 13 for the cylinder bores to securely seal around the cylinder bores.
Incidentally, the tightening pressures by the head bolts passing through the bolt holes 17 are applied to the areas or ranges A in FIG. 7, while the tightening pressures by the head bolts passing through the bolt holes 18 are applied to the areas B in FIG. 7. The surface pressures are formed by tightening the head bolts.
In case the cylinder head gasket 11 in the prior art is installed between the cylinder head and the cylinder block, in case the sealing pressures by the tightening forces of the bolts are too weak, the function for sealing the combustion gas can not be sufficiently obtained. On the other hand, in case the sealing pressures by the tightening forces of the bolts are too strong, it causes the damages to the beads or creep relaxation to the beads. Thus, the sealing characteristics of the beads can not be maintained for a long period of time. Therefore, it is required to tighten the bolts at adequate tightening pressures, so that the sealing pressures generated by the tightening pressures of the bolts can provide sufficient sealing characteristics for a long time.
For this purpose, when the cylinder head gasket 11 is installed on the cylinder block together with the cylinder head, the bolts are tightened according to the predetermined order. In addition, in order to provide the equal tightening torque to the bolts, the bolts are tightened by a torque wrench.
In the gasket 11 as shown in FIG. 7, the sealing area B to which the tightening forces of the bolts passing through the bolt holes 18 are applied in a very small as compared to the area A to which the tightening forces of the bolts passing through the bolt holes 17 are applied. Therefore, the surface pressures acted on the area A become very small relative to the surface pressures acted on the area B, so that the beads 12 in the areas B are subjected to strong tightening pressures more than necessity. Thus, the equal tightening pressures are not obtained at the gasket 11 to thereby cause the problems such that the cylinder head is slightly curved like an arch to deform the lower surface thereof and to lower the sealing ability.
In this gasket, while the engine is operated, unequal pressures are applied to the beads. Therefore, the beads are likely to creep.
In case the tightening torque for the bolts passing through the bolt holes 18 at both end areas of the gasket 11 is made less than that for the bolts passing through the bolt holes 17 at the middle area, the surface pressure formed may be made equal. However, in this case, the sealing pressures are insufficient as a whole, so that the sufficient sealing characteristics are not obtained.
In order to solve the problems, in Japanese Utility Model Publications No 6-32828 and No. 6-32829, there are formed auxiliary beads or shims at the peripheries of the holes for the cylinder bores located at both end areas of the gasket. When the head bolts are tightened at the equal tightening torque, even if the sealing areas to which the tightening forces of the head bolts are applied are different, the sealing pressures are made equal. In this case, the sufficient surface pressures are obtained while the deformation of the cylinder head is prevented.
The auxiliary beads and shims disperse large sealing surface pressures applied to the beads as the main sealing at the two end areas of the gasket. As a result, since the heights of the compressed beads become substantially equal for the whole holes for the cylinder bores in the condition that the gasket is held between the cylinder head and the cylinder block, the deformation or warp of the cylinder head abutting against the cylinder head gasket in the compressed condition is suppressed.
However, since the auxiliary bead and shim operate to disperse the sealing surface pressures, the sealing surface pressures formed around the holes for the cylinder bores at the end areas and the sealing surface pressures formed around the holes for the cylinder bores at the middle area are not equalized. By increasing the sealing surface pressures around the holes for the cylinder bores at the end areas, the sufficient sealing surface pressures are obtained at the hole for the cylinder bore located in the middle area.
As a result, a large tightening force is required. In case the cylinder head and cylinder block are formed of aluminum alloy for reducing the weight of the engine, when the cylinder head and the cylinder block receive the large tightening force, the cylinder head and the cylinder block are likely to receive damages.
Also, when an auxiliary bead or shim is formed, since the auxiliary bead or shim is arranged inside or outside the main bead, it requires a space for the auxiliary bead or shim. Since the head gasket is made smaller as the engine is made smaller, it is difficult to dispose the auxiliary bead or shim in layout.
In case the shims are used, very thin shims must be processed precisely, and attached to or disposed on a plate for the cylinder head gasket. Therefore, it requires labor and cost in processing and storing the shims to increase the manufacturing cost.
On the other hand, a material, such as plastics with heat and pressure resistance properties, is coated on a part of a bead by screen printing and the like to provide surface pressure reinforcing coating to reinforce and increase the surface pressure of the bead. This processing can prevent damage of the cylinder head formed of the relatively soft aluminum alloy.
The present invention has been made to obviate these problems, and an object of the invention is to provide a cylinder head gasket, wherein the surface pressure around the hole for the cylinder bore where the surface pressure is reduced can be reinforced at a relatively small tightening pressure.
Another object of the invention is to provide a cylinder head gasket as stated above, wherein the surface pressures around the cylinder bores can be made substantially equally.
Further objects and advantages of the invention are apparent from the following description of the invention.
A cylinder head gasket of the invention is used for an internal combustion engine with at least three cylinder bores. The gasket is formed of a first metal plate extending substantially throughout an entire area to be sealed, and a first reinforce coating. The first metal plate includes two end holes, at least one inner hole sandwiched between the two end holes, and beads surrounding the respective holes. The two end holes and at least one inner hole are arranged linearly to correspond to the cylinder bores of the engine, and have peripheral portion around the holes. The first reinforce coating is deposited around the peripheral portion of the at least one inner hole so that sealing pressures are formed substantially equally around the two end holes and at least one inner hole.
The cylinder head gasket may further includes second reinforce coatings deposited around the peripheral portions of the two end holes. In this case, the thickness of the first reinforce coating is greater than that of the second reinforce coating.
The reinforce coatings may be selected from the group consisting of epoxy resin, phenol resin, phenoxy resin, fluoroplastics and polyamideimido. A desired shape of the reinforce coating may be applied by screen printing or spraying with a mask. In case the first and second reinforce co tings are formed, the reinforce coatings may have one layer with different thicknesses or a plurality of layers to change the thickness by changing the number of layers. Especially, phenoxy resin has heat resistance over 200xc2x0 C., and in view of working ability and coating characteristics, phenoxy resin is most preferable.
In the cylinder head gasket with more than three holes for the cylinder bores arranged linearly, since the reinfore coating is formed thick or formed only around the peripheral portion of the inner hole for the cylinder bore where the tightening pressure is relatively low, the surface sealing pressure can be formed at the peripheral portion of the inner hole for the cylinder bore substantially equally to that at the peripheral portions of the end holes for the cylinder bores. Therefore, it is possible to reinforce the surface pressure for the inner hole where the sealing pressure is relatively low to improve the sealing ability.
Also, a large tightening torque is not required, and the large sealing surface pressures are not applied at the beads around the end holes. Therefore, creep relaxation and damages of the beads around the end holes are prevented to improve the durability.
Since the creep relaxation and damages of the beads can be avoided, in the condition that the gasket is installed between the cylinder block and the cylinder head, the gasket can keep the thickness equally around the all holes for the cylinder bores. Therefore, the deformation of the cylinder head abutting against the gasket in the compressed condition is suppressed.
When the reinforce coatings are made by a screen printing, even if the coatings have complicated shapes, the coatings can be made easily. Further, the thicknesss of the coatings can be changed easily by multiple printings, and the coatings with adequate thicknesses according to the portions of the gasket can be formed easily.
In the maltiple printings, the reinforce coatings can be formed in various shapes and thicknesses. Also, it is possible to provide a multiple structure with different materials.
Incidentally, the gasket of the invention may be formed of one or a plurality of metal plates. Also, in addition to the above reinforce coating, the gasket may have another coating.
In the cylinder head gasket, the first and second reinforce coatings are formed on and around the respective beads. The first and second reinforce coatings may be formed of a plurality of coating layers deposited on two sides of the first metal plate. The first and second reinforce coatings have heatproof characteristic and compressibility to strengthen surface pressures of the beads.